Playing a game of Gears of War against some dude who routinely kicks my ass in the game, and winning with sheer guile and trickery.
Basically, he'd play a sniper, and I'd pop out and place a few shots in his direction, rebase and repeat until he ran out of sniper rifle ammo. So he had to try and rush me with the Lancer. He was climbing a flight of stairs where I was taking cover behind this short wall at the top, and then one of those lucky "once in a lifetime" shots happened. I popped out to get a good look, and his head was right THERE, in my shotgun reticule at close range.
Trigger pulled, instant win. Most awesome win of the game, bar none.